Shale is by far the most abundant sedimentary rock. Although strictly speaking shale is a rock, it is relatively weak and soft, and engineers consider it to be a soil. However, shale differs from most soils in a number of ways that affect its engineering uses. Shale typically is dense and has a very low content of voids, causing it to be relatively impermeable to water. Second, shale typically has a finely layered bedding structure and readily shears along these planes. Third, shale used as a fill soil gradually degrades and reverts to clay, causing delayed difficulties and failures. While engineers may be content to call shale a soil, there are abundant reasons why it also should carry a designation as shale.
The cause of the finely layered ‘‘shaley structure’’ of shale is compression under large amounts of overburden coupled with geological time. This process of densification is called consolidation, similar to the consolidation that occurs in clay under a foundation load but to a much larger degree. Thus, whereas the void content of soils is measured in tens of percent, the void content of a shale may be only a few percent. As shale consolidates, the clay mineral platelets tend to become oriented flat and parallel to one another and create the shale structure.
Claystones and siltstones are rocks having a similar grain-size composition that are not so thinly bedded.
Shales and Landslides
The planes of weakness in shales are highly conducive to landslides, particularly if the rock layers have been tilted from deformations of the Earth’s crust.
This occurs as a result of plate collisions, faulting, folding, and mountain building.
Sometimes the Earth’s crust has been compressed so that rock layers are pushed at the edges to form a series of symmetrical folds like ripples on water. Some very large and tragic landslides are attributed to shale.
Shale Rebound from Unloading
Another property of shale that contributes to a catalogue of lamentable surprises is a tendency to rebound and expand when pressure is removed by excavations.
Expansion is triggered if water is available to weaken the electrostatic bonding in the clay. Excavations in shale therefore should not be allowed to remain open any longer than absolutely necessary, and should be kept dry. This can be accomplished by use of a thin layer of asphalt or Portland cement concrete.
Geologically old shale does not contain expansive clay minerals, and while pressures from elastic rebound are relatively moderate, they can lift lightly loaded floors while leaving nearby bearing walls intact. Damages are reduced if floors and walls are kept separated. However, interior partition walls founded on a heaving floor slab can be troublesome. Shale deposits that are Paleozoic or older generally have been chemically modified or ‘‘deweathered’’ to more stable clay minerals.
Geologically young shale, younger than Paleozoic, often contains expansive clay minerals, or minerals that expand on wetting and shrink on drying. Tests described later in this book can determine the presence and expansive potential of expansive clay.
Expansive clays constitute one of the most difficult engineering problems, inflicting extensive damages on highways, floors, walls, and foundations. Several control measures may be used. A common approach is to try and keep the water out, but this may be difficult because of the high affinity of an expansive clay for water. The attraction can be reduced with chemical stabilization, which is most commonly accomplished using hydrated chemical lime, Ca(OH)2.
Weathering of Shale
Shale weathers by simply disintegrating back into clay. As weathering is most intense near the ground surface, several distinctive layers may form that are called a ‘‘soil profile.’’ A typical soil profile consists of an organic rich topsoil that is weak and should not be used in engineering construction, over a subsoil in which clay has been concentrated.
Geomorphology of Shale
Geomorphology is the study of landforms as they relate to geological materials and processes. Streams cutting headward into shale are like fingers randomly reaching out for water, and create a dendritic drainage pattern. Shale soils have a low permeability and are readily erodible, which results in a finely sculptured drainage pattern.
Shale as a Construction Material
The hardness of dense shale can be deceiving, because when it is broken up and used as fill it may slowly absorb water and soften into clay. The moisture content of unweathered shale can be as low as 2 or 3 percent compared with 25 to 50 percent or more in more ordinary clayey soils. Gradual softening of shale can result in slope failures mysteriously occurring years after an embankment is completed. Considerable caution should be exercised before using an untried shale in construction.